1. Field of the Invention
The present invention relates to an optical recording medium processing apparatus for irradiating with a laser beam, and accessing, a recording medium, such as a magneto-optical (MO) disk, and in particular, to the automatic power control (APC) of a laser beam by such an apparatus.
2. Related Arts
A common optical recording medium processing apparatus for accessing information recorded on a recording medium, such as a CD or a magneto-optical disk (hereinafter referred to as an MO disk), employs a semiconductor laser as a light source. To reproduce information recorded on the recording medium, the recording medium is irradiated by a laser beam, and light returning from the recording medium, or transmission light, is detected. In particular, to reproduce data recorded on an MO disk, on the surface of which there are at the least pit areas and magneto-optical signal areas, which are information signal areas from which information is optically read, light emitted by a semiconductor laser is used to irradiate the MO, and the level of the light returned by the MO is detected for the pit areas, while the polarization element of the returned light is detected for the magneto-optical signal areas.
A pit area is an area wherein information is recorded using plane and recessed portions formed on the substrate of the recording medium. The level of the returning light fluctuates in conformity with phase differences in the returning light occasioned by the height and depth of the plane and recessed portions, and therefore, the information recorded in a pit area can be reproduced by detecting the level of the returning light. A magneto-optical signal area, however, is an area wherein information is recorded using the directions of the magnetic field alignments of individual magnetic domains that are formed in a magnetic film on the recording medium. The direction in which the returning light (or transmission light, when the light passes through the information signal area) is polarized differs depending on the direction of the magnetic fields, and thus, the information recorded in the magneto-optical signal area can be reproduced by detecting the polarization element contained in the returning light (or the transmission light).
In operation, if a constant current is used to drive a current-driven semiconductor laser, the level of light emitted by the semiconductor laser is reduced as the temperature of the semiconductor laser rises. However, since for practical reasons the intensity of a laser beam that is emitted must be maintained at substantially a constant level, regardless of changes in temperature, so-called automatic power control (hereinafter referred to as APC) by monitoring the level of emitted light is employed in order to drive the semiconductor laser. The APC method monitors the part of the emitted light and controls the drive current supplied to the semiconductor laser so that a voltage level of the emitted light is equal to the a predetermined reference level.
FIG. 9 is a block diagram illustrating a conventional optical recording medium processing apparatus for using APC. In FIG. 9, a laser beam, which is emitted by a semiconductor laser 1 in an optical head 10, passes through a coupling lens 2 and a round correction lens 3, and enters a beam splitter 4. Light returning from the beam splitter 4 is collected at a lens 5, and is received at a light receiving element 6 for monitoring the emitted light. Light passing through the beam splitter 4 is guided by a carriage 11 to a location which it irradiates on an MO disk 13, which is rotated by a spindle motor 12. Then, a magnetic head 14 is used to record information in the magneto-optical signal area of the MO disk 13. The light returning from the MO disk 13 is reflected by the beam splitter 4, and the reflected light is transmitted through a cylindrical lens 7 and a compound element 8, whereat astigmatism is induced, and is received at a light receiving element 9 for the detection of a reproduction signal.
A current output by the light receiving element 6 is converted by a voltage conversion amplifier 24 to an emitted light monitor signal S10, which has a voltage level corresponding to an output current value. The signal S10 is transmitted to a comparator 22, which compares the voltage of the emitted light monitor signal S10 with reference voltage Vref1, supplied by a reference voltage generator 21, and outputs, to a drive circuit 23, a drive signal S11 having a voltage which is adjusted in accordance with the comparison results. Subsequently the drive circuit 23 supplies, to the semiconductor laser 1, a drive current which corresponds to the voltage of the received drive signal, and the semiconductor laser 1 emits a laser beam which has an intensity corresponding to the value of the received drive current. In this manner, a laser beam having a constant intensity is emitted by the semiconductor laser. When a voltage-driven semiconductor laser is employed, a voltage corresponding to a drive signal is supplied to the semiconductor laser, which emits a laser beam having an intensity which corresponds to the voltage level.
In addition, pulse emission is induced by superimposing on a current supplied to the semiconductor laser 1 a high-frequency current having a satisfactorily large amplitude. Pulse emission is employed in case that a method for recording information in the magneto-optical signal area is light modulation magneto-optical recording, while DC emission is employed in case of magnetic modulation magneto-optical recording. During pulse emission, since the semiconductor laser 1 is oscillated in a multi-mode by superimposing on a drive current a high-frequency current, the emitted light monitor signal S10 and the signal S11 include wide band frequency elements.
When these signals spreads on a frequency band, laser noise tends to be increased and the semiconductor laser 1 to be easily oscillated. To resolve this shortcoming, the semiconductor laser 1 and the light receiving element 6 for the monitoring of emitted light are connected together using the shortest possible wiring.
However, when the semiconductor laser 1 and the light receiving element 6 are located too near each other, it is decreased the degree of freedom available in the arrangement of optical components in the optical head 1.
Further, even if the level of the light emitted by the semiconductor laser 1 is constant, the level of the light returning from the recording medium is not always so. Although it would be ideal, to reproduce the information recorded in an information signal area, such as the magneto-optical signal area, for the level of the returning light to be constant, however, in actuality, while the level of the emitted light may be constant, the level of the returning light is not. When the level of the returning light, which should originally be constant, is varied, a reproduction signal is adversely affected and the signal to noise ratio (C/N ratio) is deteriorated. In particular, when the recording density in the magneto-optical signal area is to be increased, the level of the returning light which corresponds to one domain in the magneto-optical signal area is reduced, and the C/N ratio for the reproduction signal in the magneto-optical signal area is greatly deteriorated. Therefore, a further reduction of noise is desirable.
It is, therefore, one objective of the present invention to provide an optical recording medium processing apparatus which can reduce the noise included in the reproduction signal for the information signal area.
It is another objective of the present invention to provide an optical recording medium processing apparatus with which the degree of freedom available in the arrangement of optical components in the optical head can be increased.
To achieve the above objectives, according to a first arrangement of the present invention, it is provided an optical recording medium processing apparatus for irradiating a laser beam to a recording medium including an information signal area on which information is recorded at least optically and a pit area on which information is recorded by using pits to access the recording medium comprising:
a light source for emitting a laser beam having a level corresponding to a control value which is instructed;
a first light detector for detecting light returning from the recording medium;
a signal detecting section for detecting whether a detection signal corresponding to the returning light detected by the first light detector is from the information signal area or is from the pit area; and
a first power controller for controlling the control value based on the level of the detection signal to instruct the control value to the light source in case that the signal detector detects the detection signal from the information signal area.
Since APC for the information signal area is performed based on the level of the light returning from the recording medium so as to be substantially constant, the signal-to-noise ratio (C/N ratio) can be improved.
In addition to the first arrangement, according to a second arrangement of the present invention, it is provided the optical recording medium processing apparatus further comprising:
a second light detector for detecting monitor light which is a part of the light emitted from the light source; and
a second power controller for controlling the control value based on a level of a monitor signal corresponding to the monitor light to instruct the control value to the light source in case that the signal detector detects the detection signal from the pit area.
The APC for the pit area, the level of the returning light wherefrom fluctuates, is performed based on the level of a monitor signal corresponding to the light emitted by the light source.
In addition to the first arrangement, according to a third arrangement of the present invention, it is provided the optical recording medium processing apparatus further comprising:
a second power controller for instructing a predetermined control value to the light source in case that the signal detector detects the detection signal from the pit area.
With this arrangement, APC is not performed for the pit area, the level of the returning light wherefrom fluctuates, and the level of the laser beam is controlled in accordance with a predetermined control value. Therefore, the second light detector, for detecting monitor light, in the second arrangement can be removed from the optical head. As a result, the degree of freedom available in the arrangement of optical components in the optical head is increased, and a compact optical head can be constructed.